Various types of computing, entertainment, and/or mobile devices can be implemented with a near-eye transparent or semi-transparent display through which a user of a device can view the surrounding environment. Such devices enable a user to see through the transparent or semi-transparent display of the device to view the surrounding environment, and also see images of virtual objects (e.g., text, graphics, video, etc.) that are generated for display to appear as a part of, and/or overlaid upon, the surrounding environment.
These devices often utilize optical waveguides to replicate an image, e.g., produced by a display engine, to a location where a user of a device can view the image as a virtual image in an augmented reality environment. As this is still an emerging technology, there are certain challenges associated with utilizing waveguides to display images of virtual objects to a user.
In near-eye display devices that utilize optical waveguides, such as head mounted displays (HMDs), heads up displays (HUDs) and other types of imaging devices, light propagates through the optical waveguide only over a limited range of internal angles. Light propagating at some non-zero angle of incidence to a surface of the waveguide will travel within the waveguide, bouncing back and forth between the surfaces, so long as the angle of incidence with respect to the surface normal is greater than some critical angle associated with the material from which the optical waveguide is made. For example, for BK-7 glass, this critical angle is about 42 degrees. This critical angle can be lowered slightly by using a reflective coating, or by using a material having a higher index of refraction, which is typically more expensive.
Regardless, the range of internal angles over which light will propagate through an optical waveguide does not vary much, and for glass, the maximum range of internal angles is typically below 50 degrees. This typically results in a range of angles exiting the waveguide (i.e., angles in air) of less than 40 degrees, and typically even less when other design factors are taken into account. For example, in optical waveguides that include an intermediate component used for pupil expansion, which is distinct from the input-coupler and output-coupler of the waveguide, the intermediate component typically limits the diagonal field-of-view (FOV) that can be supported by an optical waveguide based display to no more than 35 degrees.